Influenza virus is an RNA virus having negative-strand RNA as its genome. Frequent mutations occur in the phenotype or genomic nucleotide sequences of influenza virus, and hence the virus occasionally give rises inter-specie infection.
Influenza A virus is a major human and animal pathogen, which periodically causes a global pandemic and may result in a catastrophic loss of life. The recent emergence in Asia of avian influenza related to highly pathogenic forms of the human virus has highlighted the urgent need for new effective treatments (Non-patent Document 1).
Most current influenza drugs target either hemagglutinin (HA) or neuraminidase (NA) on the virus surface. These two major antigens are present on the virion surface (Non-patent Document 2), and 16 different HA subtypes and 9 different NA subtypes have been identified (Non-patent Document 3). Depending on the combination of these subtypes, the type of influenza virus (e.g., H1N1, H3N2, H5N1) is identified. For example, oseltamivir (commercially available under the name “Tamiflu”) and zanamivir (Relenza) are NA inhibitors and prevent virus particles being released from infected cells (Non-patent Documents 4 to 7). Oseltamivir is a drug stockpiled with a budget of several billion dollars in response to the new influenza epidemic in Asia. However, oseltamivir-resistant influenza is already emerging, and this drug is of limited use in children due to its side effects. Amantadine, an anti-influenza drug, targets the M2 protein (viral proton channel) (Non-patent Document 8), while another drug has been developed based on the three-dimensional structure of influenza M2 protein (Non-patent Document 9). However, in the case of these drugs targeting the M2 protein, a single residue mutation in M2 is sufficient to confer resistance to the virus, which may render the drugs useless against many strains. Moreover, influenza B virus does not have M2. Both oseltamivir and amantadine target proteins with a single known function and substantial sequence variation between viral strains. Thus, there is a need to develop new lead molecules disrupting other processes in the viral life cycle.
Influenza virus RNA polymerase plays an important role in virus multiplication after infection in humans, and hence can be used as a target for anti-influenza virus agents. However, none of the current medications targets the viral RNA polymerase. The viral RNA polymerase plays an extensive role in viral replication (Non-patent Document 10). This polymerase consists of a heterotrimeric complex with a total molecular weight of approximately 250 kDa, composed of polymerase acidic subunit (PA), polymerase basic subunit 1 (PB1) and polymerase basic subunit 2 (PB2) (Non-patent Document 11). All of these three subunits are required for both transcription and replication (Non-patent Document 12). Until now, very little information has been reported for the structure of the RNA polymerase (Non-patent Documents 13 to 15). In addition, none of the current reports mentions techniques for large-scale expression of the RNA polymerase, which are the key to RNA polymerase studies.